<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script type="text/javascript">
    var  a=2020;
    if (a%4==0&&a%100!=0 || a%400==0){
        alert("是闰年");
    }else {
        alert("不是闰年");
    }
</script>
</body>
</html>